I notice that virtually all of the pre-assembled router tables include at least a front miter slot. Is there any advantage to having one?
It's a personal choice. Some people prefer them; some don't.
One can easily accomplish the same cuts without the miter slot or the t-track or the combo track.
Moving in a straight line can be done with a miter gauge in the miter slot. It can also be done by a sled running along the table edge.
Clamping can be done with a t-track. For example, one can clamp a featherboard to the t-track. Clamping can also be done directly onto the table top, without the need for a t-track.
The miter track and the t-track are just conveniences, not necessities.
Similarly, putting a hole in the table top to mount a starter pin is a convenience, not a necessity. A starter "pin" doesn't have to be a pin at all. A piece of lumber clamped to the table top, with a corner in the right position to act as a starter "pin", will do just as well.
Also, does one put in t-tracks to mount the fence? Again, they're conveniences, not necessities. Fences can be clamped to the table top.